In general, a polarizer plate is used for a light control device using a liquid crystal cell. As the liquid crystal cell, for example, a TN (Twisted Nematic) liquid crystal cell or a guest-host (GH) liquid crystal cell is used.
FIGS. 15A and 15B are schematic drawings each showing the principle of operation of a conventional light control device. The light control device mainly comprises a polarizer 1 and a GH cell 2. Although not shown in the drawings, the GH cell 2 is sealed between two glass substrates, and comprises an operating electrode made of ITO (Indium tin oxide) or the like, and a liquid crystal orientation film such as a polyimide film or the like (this applies to the description below). Also, a positive liquid crystal molecule 3 and a positive dichroic dye molecule 4 are sealed in the GH cell 2.
The positive dichroic dye molecule 4 has anisotropy of light absorption, and is, for example, a positive (p-type) dye molecule which absorbs light in the long-axis direction of the molecule. The positive liquid crystal molecule 3 has, for example, positive anisotropy of dielectric constant.
FIG. 15A shows the state (no-voltage-applied state) of the GH cell 2 with no voltage applied. Incident light 5 is transmitted through the polarizer 1 to be linearly polarized. In FIG. 15A, the polarization direction coincides with the molecular long-axis direction of the positive dichroic dye molecule 4, and thus light is absorbed by the positive dichroic dye molecule 4 to decrease the light transmittance of the GH cell 2.
As shown in FIG. 15B, with the voltage applied to the GH cell 2, the positive liquid crystal molecule 3 is oriented in the direction of an electric field, and thus the molecular long-axis direction of the positive dichroic dye molecule 4 is perpendicular to the polarization direction of linearly polarized light. Therefore, the incident light 5 is transmitted through the GH cell 2 with substantially no absorption by the GH cell 2.
In the GH cell 2 shown in FIGS. 15A and 15B, mean light transmittance (based on light transmittance (=100%) in the air when a polarizer is added to a liquid crystal cell; this applies to the description below) of visible light increases with application of an operating voltage, as shown in FIG. 16. However, when the voltage is increased to 10 V, the maximum light transmittance is about 60%, and the light transmittance slowly changes.
In the use of a negative (n-type) dichroic dye molecule which absorbs light in the molecular short-axis direction and which is converse to the positive dichroic dye molecule 4, light is not absorbed with no voltage applied, while light is absorbed with the voltage applied.
In the light control device shown in FIGS. 15A and 15B, the ratio of absorbance with the voltage applied to absorbance with no voltage applied, i.e., the optical density ratio, is about 10. Therefore, the light control device has an optical density ratio of about 2 times as high as the ratio of a light control device comprising only the GH cell 2 without the polarizer 1.
In the use of a conventional guest-host liquid crystal cell, a dichroic dye molecule is used in a liquid crystal element, and there is thus the problem of deteriorating the dye molecule by excessive ultraviolet irradiation.
Namely, ultraviolet light is incident on the light control device from the outside through an effective optical path of an imaging device, and the dichroic dye molecule contained in the guest-host liquid crystal element is ionized (changed in physical properties) by optical decomposition or deterioration due to the incident ultraviolet light. Therefore, the color of the molecule is changed or faded to deteriorate the light absorption function basically possessed by the molecule, thereby deteriorating the light absorption effect and the driving efficiency of the guest-host liquid crystal element.
Accordingly, an object of the present invention is to provide a light control device and an imaging device suitable for effective and stable drive of a guest-host liquid crystal element.